CONTINENT (1IL. continens, from Lat. eon tincrc, to touch, from con-, together tenerc, to hold). The largest natural land division; of greater area than an island or peninsula. The outer portion of the earth is composed of two 'layers, the solid rocky crust, or 'lithosphere,' and the water areas, or 'hydrosphere.' In the early period of its history the earth may have been surrounded entirely by the hydrosphere, but at present, and, so far as known, in all geological ages, the crust has been folded into mountain chains, forming nuclei around which the con tinental land areas are grouped, while the waters have acemnulated in the intermediate depressions. Geographers usually recognize as continents Eurasia (•o:uprising Europe and Asia ), Africa, Australia. North America, and South America ; the two Americas. however, are sometimes grouped as a single continent. al though such a classification is hardly justifiable unless Africa be included with the Eurasian continent. A sixth continent may be represented by the land areas in the Antarctic region (Tv.). It is estimated that the land constitutes about 55,000,000 square miles, or 28 per cent. of the entire surface of the earth. The continents vary widely in form, area. relief, and distribution on the globe, yet they may have many features in common. 'Usually the regions of greatest elevation are found in the interior, while along the coast line there is a gentle slope outward which, continued beneath the sea, forma a slightly submerged land strip called the 'continental shelf.' submerged the seaward edge of the shelf the slope is very rapid down to the great depths of the sea. The average altitude of the continents, according to the calculations of Lapparent, Mur ray. Pend:, Supan, and Ileiderich, is shown below: Between the form and distribution of the con tinents many interesting comparisons may be drawn. The two Americas. comprising the greater part of the land area in the New World, are triangular in shape, the apex of the one lying in the Isthmus of Panama and the apex of the other being represented by Cape horn. Both continents are bounded on the west by a long mountain system and both have a region of lower elevation in the eastern portion. The Old World, on the other hand, is composed of a sin gle triangular land area which has its base on the Arctic Sea and its apex at the Cape of Good Ilope. Here the main trend of the moml tain chains is east and west. In general, the continents that extend into or lie within the Southern Hemisphe•e—South America, Africa, and Australia—are most regular, contrasting strongly in this particular with North America and Eurasia in the Northern Hemisphere. The northern continents have a wider extension from east to west than the southern, and arc further characterized by a great group of islands lying along the southeastern coast.
That the great land areas are not stable either as to form or elevation may be regarded as established beyond doubt by geological evidence. Moreover, certain coastal regions are known at the present time to be undergoing changes of level by which land emerges above or sinks below the sea. The extent of these oscillations in past
ages can only be conjectured. Lyell's theory that there has been a constant interchange be tween the land and water areas has been ob jected to on the ground that there is no evidence that the abysmal depths of the ocean have ever been elevated ; this objection has been weakened, however, by the discovery within continental areas of deposits abysmal in character and contain ing a deep-sea fauna. The changes of level be tween the land and the sea take place very slowly, and may be caused either by gradual vertical movement of the land area or by varia tions in the level of the ocean itself. Geologists generally agree that the positions of the present continents were determined as far hack as Arehuran times. The Laurentian plateau of North America. the Brazilian highlands of South Amer ica, and the Scandinavian peninsula and Lap land in Europe are composed of crystalline rocks, and except on the margins they are bare of all sediments. These primitive lands were extended in area by the deposition of sedimen tary strata on their 'borders, and by great up heavals accompanied by foldings of the crust into mountain ranges.
The evolution of the continental lands can be studied only tentatively, and is largely con jectured from the evidence afforded by the char acters of the fauna and flora that lived in past ages. During the Cretaceous and Tertiary times the animal and plant life of South America, South Africa, and India were strikingly similar, while there was also a uniformity between the life-forms of Europe and North America. This circumstance can best be explained by the as that in these periods the continents had an east and west trend, so that Brazil. Cen tral Africa. and Lower India were united by one broad land-strip, and eastern Canada with Europe by another. Between the northern and southern continents an ocean basin extended from the isthmus of Central America eastward to the Indian Ocean, or nearly at right angle to the basin now occupied by the Atlantic. The changes by which the continents assumed their present form took place gradually and were accomplished by a slow depression of portions of the laud and by eneroaehment of the sea. It is probable that certain regions for a long time remained above sea-level as large islands, the unsubmerged remnants of which still exist, for example, in the Cape Verde and Canary islands. in the British Isles. and in Madagascar. These changes were doubtless completed before the ap pearance of mankind: at least within historical times, so far as is known, there has been no marked alteration in the form of the continents.
BIBLIOGRAPHY. Suess. Des Antlitz der Erde Bibliography. Suess. Des Antlitz der Erde ; Neumayr. Erdgesehichte (Leipzig, 1895) ; Mill, The Realm of 'Nature (New York. 1S95) ; Mill, The International Geography (New York, 1900). See GEOLOGY;